During the operation of a motion picture film projector, film is fed through a gate that usually is located in line with the light source and the synchronized shutter. The opening and closing of the shutter is synchronized so that it corresponds to the rate of the advancement of the film through the gate. In the prior art, the shutter usually resembles one of several shapes and is wholly opaque over a portion of the surface. For example, the shutter may be a semicircle; have two or more opaque blades; or may be barrel shaped. The prior art shutters usually are comprised of a metallic material, typically steel or aluminum. Each of these prior art shutter designs are effective for blocking the light when they are synchronized to the film, but none of these shutters address the phenomena known as "thermal shock defocusing" which is film warping caused by the rapid changes in temperature/heat energy in the film when the shutter alternates between blocking and unblocking of the light.
Specifically, in the prior and present art, motion picture film contains an emulsion on one side which has a different rate of thermal expansion and contraction than the substrate of the film on which it is located. The emulsion on the film, which is more opaque than the substrate, absorbs energy and transforms it into heat more readily than the substrate. Thus, when the film is exposed to the light source, the deferential in the thermal expansion between the emulsion on the film and the substrate itself, causes the film to bow. As a result, the emulsion expands, causing the film to move from its initial position and to be drawn towards the light source, moving the film from the optimum focus plane.
However, when the light is intercepted by the shutter, any further movement of the film towards the light source comes to a halt. In other words, when there is no light on the film, the energy absorption cannot take place. Instead there is a relative loss of heat transfer which causes the film to recede or recover slightly. Thus, as each successive exposure occurs, the film surface stands somewhere between its previous position during the last frame and the zero plane. Therefore, as the film advances through the gate, there is a constantly increasing and decreasing deformation during each exposure causing the distance of the emulsion surface relative to the lens to constantly change.
FIG. 1 shows the correlation between the movement at the center of the film surface and particular instances in the frame cycle. The same movements occur at a lessor degree at points not at the center of the film surface. Thus, then thermal shock produces deterioration of the image focus during the projection cycle.
Many articles have been written in the prior discussing the deformation caused by the light source as the film moves through the gate in the motion picture projector. In one such article entitled, "Modulated Air Blast for Reducing Film Buckle," by Willy Borberg, in the August 1952 issue of the Journal of the SMPTE, the displacement at the frame center of the film is discussed, as well as one prior art method, for trying to remedy this problem. Many different methods have been tried in the prior art to overcome this thermal shock defocusing. In this article, an apparatus that air blasts the film to cool it is disclosed. Specifically, front and rear jets provided opposing air forces to produce a force for positioning the film. While this method allowed the film to be kept within acceptable focus parameters such that there was improvement in screen image definition, this method proved unreliable and inconsistent due to several variables such as the film history and its condition, different light levels and air-hose pressure changes. The air-jet noise also was considered objectionable. Further, as screens and films became larger and larger, even when the air was pulsed, the displacement of the perfect focus position from the center to the side of the frame was too large to allow improved focus definition over the entire screen.
A variation on the previous prior art method was the use of a constant pressure air chamber, either behind or in front of the film plane ("CineFocus"). Although this air pressure chamber provided resistance to the thermal drift deformation, it did not address the cyclic action of the shutter.
Another prior art method that was attempted to overcome the problem with thermal drift deformation was the development of a curved gate. For example, if the gate of a projector is curved along the axis of the film travel, it would be difficult for any of the emulsion on the film to curve transversely along an axis parallel to the frame line. Specifically, the curved gate made it harder for the film to bow out during thermal shock and minimized the common left-to-right differential focus problems common in flat gate projectors. Although the curved gate improved film rigidity, it did not prevent the negative drift from occurring especially at higher operating wattages.
Other shutters that were used in the prior art did not address the problem of the thermal drift deformation. For example in three blade shuttered projectors, while the flicker rate of the film was increased to a high enough rate so that the flicker was above the perception of most people, the shutters actually made the negative drift problem worse. Specifically, since the efficiency of the light that passes through the shutter was decreased, it necessitated the use of even higher wattage lamps to attain the same light levels, which subsequently aggravated the "thermal shock defocusing" problem.
Another prior art method that was attempted to reduce thermal drift deformation was the use of thin film optical coatings directly on the film to remove heat created by the incident light by placing a heat filter in the light path. These thin optical coatings, such as infrared reflecting, UV reflecting, and other similar type thin film coatings, allowed most of the visible light to pass through the filter while blocking some of the unwanted energy, thus minimizing the overall incident heat on the film. However, this method did not address the heat energy effect on the film as a result of the cycling of the shutter apparatus.
Another prior art method that was used in connection with reduction type projectors which are used to project mask patterns (reticles) onto surfaces of semiconductor substrates is described in U.S. Pat. No. 5,323,208 entitled Projection Exposure Apparatus. In this prior art method, the reduction-type projector utilized a stationary filter having optical coatings in or near the lens apparatus. The optical coatings used on the stationary filter were used predominately as phase shifters to strengthen the illumination and focus upon a very small area of the substrate. While this method was successful for reduction type projected images having relatively small focal lengths, it would not be successful in motion picture projectors which have an extremely long focal length. Further, insofar as the filter was in the lens and not the film plane, the problem with thermal drift deformation would not be affected.
None of the prior art methods or devices attempted to use any of the optical coatings that have been developed in the past thirty years in connection with the shutter because placing and maintaining optical coatings on the shutters existing in the prior art film projectors, would be extremely difficult and cost prohibitive. Specifically, as most classical/current projector designs utilize large diameter or conical shaped metal shutters, creating an equivalent design from a visibly transmissive substrate would be impractical, if not incredibly difficult. For example, due to the high centrifical forces being applied to the shutter (shutters typically rotate at 1440 to 3600 revolutions per minute), the mechanical strength characteristics of current suitable optical materials would not support most existing projector shutter designs.
In addition to the mechanical material strength constraints, the cost of manufacturing such a large piece (possibly vacuum formed for a conical design) of optical quality, high temperature glass would be prohibitive. In addition, the increased complexity and relative low batch yield in the optical coating process, would make the manufacture of such an item impractical.
Thus, it is an object of the present invention to develop a cost effective motion picture projector that is capable of addressing and minimizing the thermal drift deformation present in film projectors.
It is also another object of the present invention to minimize the thermal drift deformation present in, and increase the focusing abilities, of a motion picture projector.
It is yet a further object of the present invention to minimize the thermal drift deformation that is present in any projection device in which heat creates a negative effect due to cycling of a medium through an application of radiant energy, visible or otherwise.
It is another object of the present invention to significantly reduce the effect of thermal drift deformation, for a given shutter size, in an easy and cost effective (elegant) manner.